1. Field of the Invention
This invention relates to a method for synthesizing metallic films and nanopores on porous substrates. More specifically, the invention relates to palladium (Pd) based alloys on ceramic supports to create inorganic membranes containing nanopores, and the use of the membranes.
2. Background of the Invention
The production of hydrogen is widely practiced in large scale processes comprising steam reforming, water gas shift, CO2 removal, methanation and/or pressure-swing adsorption, giving a gas that is >95% hydrogen. However, the large-scale process is not economically feasible for distributed production of hydrogen needed for supplying proton exchange membrane fuel cells (PEMFC). Notable requirements include a compact device, rapid startup and turn up/turndown (<30 seconds startup time), and CO-free hydrogen feed (<1 ppm CO).
Distributed fuel processing technology involves the multi-step sequence of (1) auto-thermal-steam reforming (ATR); (2) water-gas-shift (WGS); and (3) preferential oxidation (PROX). The reforming step produces the hydrogen while the latter steps convert fuel cell poison carbon monoxide (CO) to carbon dioxide (CO2). In recent studies, methanol auto-thermal reforming and partial oxidation on Pd and Cu catalysts have demonstrated high productivities; but exit CO concentrations are 1-2%, so the additional step requiring a PROX reactor is not eliminated. The multi-step process is complex, requiring a wide range of conditions and prolonged startup times that do not meet the current demands previously detailed.
Because the conventional chemical process for the production of hydrogen involves a sequence of reaction, separation, and purification steps, these inorganic membranes appear to be a natural application for the production of hydrogen. Inorganic membrane reactors are a sub-class of multi-functional reactors that couple reaction and separation in a single unit to achieve higher conversion or desired product yield with reduced volume and/or energy consumption. These membranous reactors exist in two conformations: unsupported foils, comprised of thick layers of Pd and Pd alloys, or as supported thin films comprised of Pd or Pd alloys deposited on porous or dense support structures.
Recent studies have examined the use of Pd-based membrane reactors to generate a high purity hydrogen stream in a single unit, while resisting poisoning. Pd has a unique combination of high hydrogen permselectivity and flux, and it can withstand high temperatures. Furthermore, research indicates certain Pd alloys are resistant to sulfur hydroxide (H2S) poisoning that is resultant from hydrocarbon production of synthesis gas. Due to successes with the advantageous properties of these materials, the scientific and commercial communities actively pursued increased research and development opportunities. More recently, research has focused on improving the efficiency of the membranes. Efficiency is the compromise between flux, and permselectivity. Consequently, there exists a significant technical base, and there are a number of patented Pd and Pd alloy processes for producing membranous devices and methods of utilization.
These successes notwithstanding, the deployment of membranes has been deterred by several hurdles: (i) limited performance improvements, (ii) high materials cost, and (iii) unproven reliability. Previous studies involving permselective membrane reactors have involved selective removal of a reaction product to increase the overall conversion. In most cases the conversion gains in these cases have not warranted commercial development. The cost of permselective membranes, notably those that are Pd-based, can be prohibitive unless ultrathin films can be synthesized. Even if thickness reductions are realized, reliability can remain an issue as sub-micron Pd membranes suffer a loss in hydrogen permselectivity through defect formation when subjected to thermal swings.
Consequently, there is a need for a high flux, highly permselective, highly durable membrane with ultra thin Pd and Pd alloy incorporation.